AN ELECTROLYTE FOR MAGNESIUM ION BATTERIES

Abstract

There is a liquid electrolyte composition comprising: i) a magnesium salt comprising a trifluoromethane sulfonate anion; ii) an additive comprising an organic halide salt, an inorganic halide salt or a mixture thereof; and iii) a solvent comprising one or more ethers, wherein the organic halide salt comprises a halide anion and a cation selected from an optionally substituted quaternary ammonium or a three to nine membered N-heterocyclic cation, and the cation comprises at least one protonated nitrogen capable of dissociating the trifluoromethane sulfonate anion from the magnesium salt, and wherein the total concentration of cations of the inorganic halide salt and magnesium ions of the magnesium salt divided by the concentration of anions of the inorganic halide salt is greater than 1 in the electrolyte composition. There is further provided an electrochemical cell comprising a) a positive electrode; b) a magnesium negative electrode; and c) the electrolyte composition as described herein, wherein the positive electrode and the magnesium negative electrode are in fluid communication with the electrolyte.

Claims

1. A liquid electrolyte composition comprising: i) a magnesium salt comprising a trifluoromethane sulfonate anion; ii) an additive comprising an organic halide salt, an inorganic halide salt or a mixture thereof; and iii) a solvent comprising one or more ethers, wherein the organic halide salt comprises a halide anion and a cation selected from an optionally substituted quaternary ammonium or a three to nine membered N-heterocyclic cation, and the cation comprises at least one protonated nitrogen capable of dissociating the trifluoromethane sulfonate anion from the magnesium salt, and wherein the total concentration of cations of the inorganic halide salt and magnesium ions of the magnesium salt divided by the concentration of anions of the inorganic halide salt is greater than 1 in the electrolyte composition.

2. (canceled)

3. The electrolyte composition of claim 1, wherein the concentration of the magnesium salt is from 0.01 M to 2.5 M.

4. The electrolyte composition of claim 1, wherein the molar ratio of the magnesium salt to the organic halide salt is from 10:1 to 1:10.

5. (canceled)

6. The electrolyte composition of claim 1, wherein the N-heterocyclic cation is selected from the group consisting of: an optionally substituted three-membered heterocyclic structure, an optionally substituted four-membered heterocyclic structure, an optionally substituted five-membered heterocyclic structure, an optionally substituted six-membered heterocyclic structure, an optionally substituted seven-membered heterocyclic structure, an optionally substituted eight-membered heterocyclic structure, and an optionally substituted nine-membered heterocyclic structure.

7. The electrolyte composition of claim 6, wherein the heterocyclic structure comprises 1, 2, 3, 4, 5, 6, 7, 8, or 9 heteroatoms, said heteroatoms independently selected from the group consisting of nitrogen, oxygen, and sulfur.

8. (canceled)

9. The electrolyte composition of claim 1, wherein the organic halide salt or inorganic halide salt is selected from the group consisting of fluoride, chloride, bromide and iodide.

10. The electrolyte composition of claim 1, comprising a mixture of at least two or more of said organic halide salts, said organic halide salts being distinct from each other.

11. (canceled)

12. The electrolyte composition of claim 1, wherein the organic halide salt comprises a quaternary ammonium cation, said cation comprising a structure of N.sup.+ R.sup.1R.sup.2R.sup.3R.sup.4, wherein each of R.sup.1, R.sup.2, R.sup.3, and R.sup.4 may be the same or different and wherein each of R.sup.1, R.sup.2, R.sup.3, and R.sup.4 is independently an optionally substituted alkyl group.

13. The electrolyte composition of claim 1, wherein the organic halide salt is 1-ethyl-3-methylimidazolium chloride or tetrabutylammonium chloride.

14. The electrolyte composition of claim 1, wherein the concentration of the organic halide salt or the concentration of the inorganic halide salt is from 0.01 M to 10 M.

15. (canceled)

16. The electrolyte composition of claim 1, wherein the cations of the inorganic halide salt are lithium ions, sodium ions, cesium ions, magnesium ions, barium ions or aluminum ions.

17. (canceled)

18. The electrolyte composition of claim 1, wherein the inorganic halide salt is selected from the group consisting of lithium chloride (LiCl), sodium chloride (NaCl), cesium chloride (CsCl), magnesium chloride (MgCl.sub.2), barium chloride (BaCl.sub.2) and aluminum chloride (AlCl.sub.3).

19. (canceled)

20. The electrolyte composition of claim 1, wherein the total concentration of the cations of the inorganic halide salt and magnesium ions of the magnesium salt divided by the concentration of anions of the inorganic halide salt in the electrolyte composition is in the range of greater than 1 to 5.

21. (canceled)

22. (canceled)

23. The electrolyte composition of claim 1, wherein the concentration of the magnesium trifluoromethanesulfonate is from 0.01 M to 1.5 M.

24. A liquid electrolyte composition consisting essentially of: i) a magnesium trifluoromethanesulfonate; ii) 1-ethyl-3-methylimidazolium chloride, tetrabutylammonium chloride, or a mixture thereof; and iii) a solvent comprising 1,2-dimethoxyethane.

25. The electrolyte composition of claim 24, wherein the concentration of the magnesium trifluoromethanesulfonate salt is 2.5 M.

26. A liquid electrolyte composition consisting essentially of: i) magnesium trifluoromethanesulfonate; ii) magnesium chloride; and iii) a solvent comprising 1,2-dimethoxyethane, wherein the magnesium ions and the chloride ions are present in the electrolyte composition in a [Mg.sup.2+]:[Cl.sup.−] ratio of from >1:1 to about 5:1.

27. The electrolyte composition of claim 26, wherein the concentration of the magnesium trifluoromethanesulfonate salt is 1.5 M.

28. The electrolyte composition of claim 1, wherein the solvent does not comprise water.

29. An electrochemical cell comprising: a) a positive electrode; b) a magnesium negative electrode; and c) a liquid electrolyte composition comprising: i) a magnesium salt comprising a trifluoromethane sulfonate anion; ii) an additive comprising an organic halide salt, an inorganic halide salt or a mixture thereof; and iii) a solvent comprising one or more ethers, wherein the organic halide salt comprises a halide anion and a cation selected from an optionally substituted quaternary ammonium or a three to nine membered N-heterocyclic cation, and the cation comprises at least one protonated nitrogen capable of dissociating the trifluoromethane sulfonate anion from the magnesium salt, and wherein the total concentration of cations of the inorganic halide salt and magnesium ions of the magnesium salt divided by the concentration of anions of the inorganic halide salt is greater than 1 in the electrolyte composition, wherein the positive electrode and the magnesium negative electrode are in fluid communication with the electrolyte.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0119] FIGS. 1a and 1b are photo images of electrolyte solutions consisting of (a) 0.5 M Mg(OTf).sub.2 and (b) 0.5 M Mg(OTf).sub.2+0.15 M, 0.3 M, 0.6 M and 1.0 M of EMImCl in monoglyme.

[0120] FIGS. 2a and 2b are photo images of electrolyte solutions consists of (a) 0.5 M Mg(OTf).sub.2 and (b) 0.5 M Mg(OTf).sub.2+0.3 M TBAC in monoglyme.

[0121] FIGS. 3a and 3b are photo images of electrolyte solutions consisting of (a) Mg(OTf).sub.2-MgCl.sub.2 electrolyte solution at various [Mg2+]: [Cl.sup.−] molar ratios after stirring at 60° C. for 24 hours and (b) 1.5 M Mg(OTf).sub.2 with MgCl.sub.2.

[0122] FIG. 4a is an illustration of the strong interaction between Mg(OTf).sub.2 and EMImCl, which may result in the increased solubility of Mg(OTf).sub.2 in ether solvents.

[0123] FIG. 4b is an illustration of the strong interaction between Mg(OTf).sub.2 and TBAC, which may result in the increased solubility of Mg(OTf).sub.2 in ether solvents.

[0124] FIG. 5 is a representation of an exemplary 2032 coin cell which may be assembled. The asymmetric cell utilizes a carbon coated aluminium foil as the working electrode and a magnesium disk as a counter electrode.

[0125] FIG. 6 is a diagram showing the cell voltage measured during the galvanostatic plating/stripping for Mg(OTf).sub.2-EMImCl electrolyte solution.

[0126] FIG. 7a is a voltage profile of 0.5 M Mg(OTf).sub.2+0.3 M EMImCl in monoglyme electrolyte. The legend indicates the cycle number of each plot.

[0127] FIG. 7b is a voltage profile of Mg//Al—C cell using 0.3 M Mg(OTf).sub.2+0.3 M TBAC in monoglyme electrolyte. The legend indicates the cycle number of each plot.

[0128] FIG. 7c is a voltage profile of Mg//Al—C cell using 0.3 M Mg(OTf)2+0.2 M MgCl2 in monoglyme electrolyte. The legend indicates the cycle number of each plot.

[0129] FIG. 8a is a plot showing plating/stripping Coulombic efficiency of Mg anode in electrolytes containing 0.3 M EMImCl with Mg(OTf)2 salt concentration varied from 0.25 M to 1 M.

[0130] FIG. 8b is a plot showing Coulombic efficiency of Mg//Al—C cell using 0.3 M Mg(OTf).sub.2+0.3 M TBAC in monoglyme electrolyte. Mg//Al—C cells are cycled at a current density of 0.5 mA/cm.sup.2 and areal capacity of 0.1 mAh/cm.sup.2.

[0131] FIG. 8c is a plot showing magnesium plating/stripping Coulombic efficiency of Mg//Al—C cells with various [Mg2+]: [Cl.sup.−] ratios in monoglyme. Mg//Al—C cells are cycled at a current density of 0.5 mA/cm.sup.2 and areal capacity of 0.1 mAh/cm.sup.2.

[0132] FIG. 9a is a plot showing plating/stripping Coulombic efficiency of Mg anode using the electrolytes containing 0.5 M Mg(OTf).sub.2 salt with EMImCl concentrations varied from 0.15 M to 0.6 M.

[0133] FIG. 9b is a plot showing cell voltage measured during the galvanostatic plating/stripping of Al—C//Mg cell using 0.5 M Mg(OTf)2+0.6 M EMImCl in monoglyme for 400 cycles.

[0134] FIGS. 10a and 10b are plots showing cell voltage measured (a) and Coulombic efficiency (b) during the galvanostatic plating/stripping of Al—C//Mg cell using 0.5 M Mg(OTf).sub.2+0.6 M EMImCl in triglyme.

[0135] FIGS. 11a and 11b are voltage profiles of Al—Cl//Mg using 0.5 M Mg(OTf).sub.2+0.6 M EMImCl in monoglyme electrolyte at high areal capacity of (a) 0.5 mAh/cm.sup.2 and (b) 1 mAh/cm.sup.2. The legend indicates the cycle number of each plot.

[0136] FIG. 11c is a plot showing plating/stripping Coulombic efficiency of Al—C//Mg at high areal capacities of 0.5 mAh/cm.sup.2 and 1 mAh/cm.sup.2. The legend indicates the cycle number of each plot.

[0137] FIGS. 12a and 12b are plots showing Coulombic efficiency of Mg//Al—C cell cycled at (a) different current densities, and (b) at different areal capacities. All the cells were cycled using 0.5 M Mg(OTf).sub.2+0.3 M TBAC in monoglyme electrolyte.

[0138] FIGS. 13a and 13b are plots showing Mg plating/stripping Coulombic efficiency of Mg//Al—C at (a) different current density and (b) different areal capacity. Mg//Al—C cells are cycled with 0.3 M Mg(OTf)2+0.2 M MgCl2 in monoglyme electrolyte.

[0139] FIG. 14 is a plot showing cycling performance of Mg//Al—C cells using 0.3 M Mg(OTf).sub.2+0.3 M TBAC in monoglyme electrolyte measured at areal capacities of 0.1 mAh/cm.sup.2, 0.5 mAh/cm2, and 1 mAh/cm.sup.2.

[0140] FIGS. 15a and 15b are scanning electron micrographs of Al—C electrodes (a) before and (b) after Mg deposition cell using Mg(OTf).sub.2+EMImCl in monoglyme electrolyte at an areal capacity of 1 mAh/cm.sup.2. The scale bar of these figures is 20 μm.

[0141] FIGS. 16a and 16b are scanning electron micrographs showing morphology of deposited Mg film on Al—C electrode using Mg(OTf).sub.2+TBAC in monoglyme electrolyte. Mg film was deposited at current density of 0.5 mA/cm.sup.2 for 2 hours (1 mAh/cm.sup.2). The scale bar of FIG. 16a is 50 μm while the scale bar of FIG. 16b is 5 μm.

[0142] FIGS. 17a and 17b are scanning electron micrographs showing morphology of deposited Mg film on Al—C electrode using Mg(OTf).sub.2+MgCl.sub.2 in monoglyme electrolyte. Mg film was deposited at current density of 0.5 mA/cm.sup.2 for 2 hours (1 mAh/cm.sup.2). The scale bar of FIG. 16a is 10 μm while the scale bar of FIG. 16b is 1 μm.

[0143] FIGS. 18a and 18b are plots showing symmetric Mg//Mg cell cycling performance at (a) 0.5 mAh/cm.sup.2 and (b) 1 mAh/cm.sup.2 in the electrolyte of 0.5 M Mg(OTf).sub.2+0.6 M EMImCl in monoglyme.

[0144] FIG. 19 is a plot showing cell voltage measured during the galvanostatic stripping/plating of a Mg//Mg symmetric cell using 0.5 M Mg(OTf).sub.2+0.3 M TBAC in monoglyme electrolyte. The galvanostatic stripping/plating was conducted at current density of 0.5 mA/cm.sup.2 and areal capacity of 0.5 mAh/cm.sup.2.

[0145] FIG. 20 is a plot showing cell voltage measured during the galvanostatic stripping/plating of a Mg//Mg symmetric cell using 0.3 M Mg(OTf).sub.2+0.2 M MgCl.sub.2 in monoglyme electrolyte. The galvanostatic stripping/plating was conducted at areal capacity of 0.5 mAh/cm.sup.2 and current density of 0.5 mA/cm.sup.2.

EXAMPLES

[0146] Non-limiting examples of the invention will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Example 1. Solubility of the Electrolytes

[0147] Mg(OTf).sub.2 and EMImCl in Ether Solvent

[0148] Experimental results confirmed the prediction on the positive effect of 1-Ethyl-3-methylimidazolium chloride (EMImCl) additive to the performance of magnesium trifluoromethanesulfonate (Mg(OTf).sub.2)-based electrolyte. In this invention, several electrolyte compositions with formula presented by x M Mg(OTf).sub.2+y M EMImCl in monoglyme (x=0.25, 0.5, 0.75, 1; y=0.15, 0.3, 0.6, 1.0) are presented. As shown in FIGS. 1a and 1b, by adding 0.15 M, 0.3 M, 0.6 M and 1.0 M of EMImCl into 0.5 M Mg(OTf).sub.2 in monoglyme, the solution turned from turbid to clear indicating the solubility of Mg(OTf).sub.2 was increased. The increased solubility could be attributed to the strong interaction between the anion of Mg(OTf).sub.2 and the cation of EMImCl as shown in FIG. 4a.

[0149] It should be noted here that the electrolyte solutions consisting of Mg(OTf).sub.2 and EMImCl in other ethers and their mixtures are also obtainable by controlling Mg(OTf).sub.2:EMImCl molar ratio. In addition, the combination of EMImCl with other magnesium salts, including Mg(TFSI).sub.2 and magnesium perchlorate (Mg(ClO4).sub.2), was examined. These electrolytes, however, demonstrate poor electrochemical performance. It is probably due to instability of TFSI.sup.− and ClO.sup.4− anions against reduction at Mg metal surface.

[0150] Mg(OTf).sub.2 and TBAC in Ether Solvent

[0151] The solubility of Mg(OTf).sub.2 was also increased by adding 0.3M (tetrabutylammonium chloride) TBAC into 0.5M Mg(OTf).sub.2 in monoglyme (dimethoxyethane, DME), as shown in FIG. 2a and FIG. 2b. In particular, TBAC assists in the dissolution of Mg(OTf).sub.2 in ether solution and acts as a source of Cl.sup.− for the formation of electroactive species (e.g. [Mg.sub.2(μ-Cl).sub.2(DME).sub.4].sup.2+). The increased solubility could be attributed to the strong interaction between the anion of Mg(OTf).sub.2 and the cation of TBAC as shown in FIG. 4b.

[0152] Mg(OTf).sub.2 and MgCl.sub.2 in Ether Solvent

[0153] It was also found that the inorganic chloride (MgCl.sub.2) also helps to improve the solubility of Mg(OTf).sub.2 in ether solvent, by carefully controlling the molar ratio between Mg.sup.2+ and Cl.sup.− in ether solvent. FIG. 3 and Table 1 demonstrates the crucial effect of [Mg.sup.2+]:[Cl.sup.−] molar ratio in the formation of a clear electrolyte solution. It should be noted here that both Mg(OTf).sub.2 and MgCl.sub.2 have a very low solubility in ether solvent. However, the reaction between Mg(OTf).sub.2 and MgCl.sub.2 in ether solvent helps in improving their solubility. Therefore, the ratio between Mg(OTf).sub.2 and MgCl.sub.2 plays an important role in the formation of a clear and stable electrolyte solution. It may be possible that Mg(OTf).sub.2 reacts with MgCl.sub.2 and forms various species such as Mg[OTf].sub.x[Cl].sub.y[solvent].sub.z(with 0≤x, y, z≤3), which are easier to be dissolved into electrolyte solution.

[0154] In particular, electrolytes with [Mg.sup.2+]:[Cl.sup.−] ratio greater than 1 were found to be clear and stable. FIG. 3a shows that clear electrolyte solutions are obtained by controlling [Mg.sup.2+]: [Cl.sup.−] between 5:2 to 5:4. On the other hand, the electrolyte solution with [Mg.sup.2+]:[Cl.sup.−] ratio of 5:5 is unstable as salt crystals form at room temperature. As for electrolytes with [Mg.sup.2+]:[Cl.sup.−] ratio of 5:6 and 5:8, the salts were found to be incompletely dissolved, forming unclear solutions. FIG. 3b shows that with addition of 1 M MgCl.sub.2, the concentration of Mg(OTf).sub.2 can reach as high as 1.5 M in dimethoxyethane (DME) and solution still remained clear. Total concentration of [Mg.sup.2+] is 2.5 M (1.5 M from Mg(OTf).sub.2+1 M from MgCl.sub.2). The concentration of [Cl.sup.−] is 2 M. Therefore, [Mg2+]: [Cl.sup.−] is 2.5:2, which equals 1.25 and is greater than 1. Therefore, it can be seen that for electrolytes where the concentration of the cations of the inorganic halide salt and magnesium ions of the magnesium salt (which in this case is the total concentration of the Mg.sup.2+ ions in the Mg(OTf).sub.2 and the MgCl.sub.2) divided by the concentration of anions of the inorganic halide salt (which in this case is Cl.sup.−) is greater than 1 in the electrolyte composition, the electrolyte composition was a clear solution, showing that the solubility of Mg(OTf).sub.2 was increased.

TABLE-US-00001 TABLE 1 Combinations of Mg(OTf).sub.2 and MgCl.sub.2 in monoglyme as electrolyte solution for Mgion batteries Electrolyte formula [Mg.sup.2+]:[Cl.sup.−] Result 0.45M Mg(OTf).sub.2 + 0.05M MgCl.sub.2/ 5:1 Unclear solution monoglyme 0.4M Mg(OTf).sub.2 + 0.1M MgCl.sub.2/ 5:2 Clear solution monoglyme 0.35M Mg(OTf).sub.2 + 0.15M MgCl.sub.2/ 5:3 Clear solution monoglyme 0.3M Mg(OTf).sub.2 + 0.2M MgCl.sub.2/ 5:4 Clear solution monoglyme 0.6M Mg(OTf).sub.2 + 0.4M MgCl.sub.2/ 5:4 Clear solution monoglyme 0.25M Mg(OTf).sub.2 + 0.25M MgCl.sub.2/ 5:5 Clear solution monoglyme (unstable) 0.25M Mg(OTf).sub.2 + 0.3M MgCl.sub.2/ 5:6 Unclear solution monoglyme 0.1M Mg(OTf).sub.2 + 0.4M MgCl.sub.2/ 5:8 Partly dissolved monoglyme

Example 2. Fabrication of an Electrochemical Cell

[0155] The electrochemical performance of the electrolyte described herein was evaluated by fabricating a 2032 coin cell comprising the electrolyte, as illustrated in FIG. 5. Unless described otherwise, the coin-cell configuration described below was adopted for the electrochemical studies of the electrolytes described herein.

[0156] In asymmetric (Al—C//Mg) cell test, the coin-cell consists of a polished Mg disk (1.27 cm.sup.2) as a counter electrode, 2 layers of Celgard separator, Al—C disk (carbon coated Aluminum foil) (1 cm.sup.2) as a working electrode, and 25 μl of Mg(OTf).sub.2 electrolyte. In symmetric (Mg//Mg) cell tests, the Al—C disk was replaced by an Mg disk (1.27 cm.sup.2).

[0157] The asymmetric cell was galvanostatically cycled with a current density of 0.5 mA/cm.sup.2. First, an areal capacity of 0.1 tnAh/cm.sup.2 of Mg was plated onto Al—C working electrode, Mg was then stripped until the voltage reaches 1.2 V. The Coulombic efficiency (CE) was defined as the ratio of stripping capacity to plating capacity.

Example 3. Electrochemical Properties Tested with Al—C Electrodes

[0158] Mg(OTf).sub.2 and EMImCl in Ether Solvent

[0159] The reversible Mg plating/stripping was successfully demonstrated in 0.5 M Mg(OTf).sub.2+0.3 MEMImCl in monoglyme (FIG. 6). The reversible plating and stripping of Mg on Al—C foil were observed near −0.35 V and 0.35 V vs. Mg/Mg.sup.2+ (FIG. 7a) respectively. FIG. 8a and Table 2 present the electrochemical performance of the electrolyte with the concentration of Mg(OTf).sub.2 varied from 0.25 M to 1 M, and 0.3 M EMImCl.

TABLE-US-00002 TABLE 2 Coulombic efficiency and cycle life of Al—C//Mg cells with different concentration of Mg(OTf).sub.2 salt. The concentration of EMImCl is 0.3M in these electrolyte solution. Salt concentration (M) Initial CE (%) Highest CE (%) Cycle life 0.25 43.1 99.2 196 0.5 49.0 99.0 265 0.75 19.3 98.1 191 1 21.3 98.2 181

[0160] In the first cycle, Coulombic efficiency of the cell was relatively low (49%), which is due to the irreversible reduction of electrolyte components and/or contaminants (e.g. moisture). With increased cycle number, Coulombic efficiency of the cell increased significantly and reached 99% in subsequent cycles.

[0161] Among these electrolyte compositions, the cell using 0.5 M Mg(OTf).sub.2+0.3 M EMImCl in monoglyme electrolyte showed best performance. This cell delivered the highest initial Coulombic efficiency (ICE) (49%) and highest Coulombic efficiency was recorded at 99% in subsequent cycles. The longest cycle life of 260 cycles was also achieved at this composition. At higher concentration of Mg(OTf).sub.2 (above 0.5 M), the cells showed lower Coulombic efficiency, which was probably due to high viscosity of electrolyte solution and increased concentration of contaminants. Therefore, the optimum concentration of Mg(OTf).sub.2 is 0.5 M.

[0162] FIG. 9a and Table 3 present the electrochemical performance of the electrolytes consisting of 0.5 M Mg(OTf).sub.2 and EMImCl additive at 0.15 M, 0.3 M, and 0.6 M. The highest initial Coulombic efficiency of 56% was achieved at 0.15 M of EMImCl. Initial Coulombic efficiency of cells decreased at higher EMImCl concentrations. During cycling, three electrolyte compositions demonstrated high Coulombic efficiency above 98%. However, longer cycle life was achievable at the higher concentrations of additive. The cell using 0.5 M Mg(OTf).sub.2+0.6 M EMImCl in monoglyme showed longest cycle life (400 cycles) with Coulombic efficiency above 98%.

TABLE-US-00003 TABLE 3 Coulombic efficiency and cycle life of Al—C//Mg cells with different concentration of EMImCl additive Cycle EMImCl concentration (M) Initial CE (%) Highest CE (%) life 0.15 56.0 98.5 142 0.3 49.0 99.0 265 0.6 32.2 99.6 401

[0163] Herein, 0.5 M Mg(OTf).sub.2+0.6 M EMImCl in monoglyme was considered as the optimum composition for Mg plating/stripping. At this composition, the Mg plating/stripping cycles were highly reversible up to 400 cycles with a slightly increasing overpotential, from ±0.35 V in early cycles to ±0.6 V at the 400th cycle. In addition, a high ionic conductivity of 5.1 mS/cm was recorded at this electrolyte composition.

[0164] The choice of ether solvents for this electrolyte system is not limited to monoglyme. As a representative example, reversible plating/stripping of Mg in an electrolyte consisting of 0.5 M Mg(OTf).sub.2+0.6 M EMImCl in triglyme (FIG. 10) was also tested. The Mg//Al—C cell delivered an initial Coulombic efficiency of 29% and a Coulombic efficiency of 95% at 50th cycle. The Coulombic efficiency was lower than that of the cell using monoglyme solvent. This is probably due to higher viscosity and higher concentration of contaminants in triglyme compared to monoglyme. From a practical point of view, the use of higher glyme solvents (diglyme, triglyme, and tetraglyme) is preferable due to lower flammability (higher boiling point).

[0165] High Areal Capacity of Mg Anode

[0166] Towards practical application of Mg metal anode, the Al—C//Mg cells were cycled at high areal capacity of 0.5 mAh/cm.sup.2 and 1 mAh/cm.sup.2 (FIG. 11), which is close to practical battery requirements. From FIG. 11, the reversible plating/stripping of Mg at these high areal capacities was successfully achieved with 0.5 M Mg(OTf).sub.2+0.6 M EMImCl in monoglyme electrolyte. In these cases, the plating and stripping overpotential slightly increased to −0.4 V and 0.4 V, respectively. Interestingly, the cells showed high initial Coulombic efficiency of 67.2% and 74.7% at the areal capacities of 0.5 mAh/cm.sup.2 and 1 mAh/cm.sup.2, respectively. Coulombic efficiency above 95% was achieved for both cells during cycling at high areal capacities.

[0167] Mg(OTf).sub.2 and TBAC in Ether Solvent

[0168] Several electrolyte compositions with formula represented by x M Mg(OTf).sub.2+y M TBAC in monoglyme (x=0.5; y=0.15, 0.3, 0.6, 1) were examined. The experiments were conducted to evaluate the key technical performance of high-performance electrolyte systems, including Mg plating/stripping Coulombic efficiency and cycle life under different cycling conditions. The electrolyte combination consisting of 0.5 M Mg(OTf).sub.2 and 0.3 M TBAC in monoglyme shows excellent electrochemical performance in a half-cell test. Table 4 summarizes the key technical features of the designed electrolyte formula.

TABLE-US-00004 TABLE 4 The key technical features of 0.5M Mg(OTf).sub.2 + 0.3M TBAC in monoglyme electrolyte. Key Features Performance Remarks Mg plating/stripping CE 97.5% (0.5 mA/cm.sup.2 and Very high Coulombic 0.1 mAh/cm.sup.2, 500 efficiency; Max CE: 98.2% cycles) Current density 0.25-4 mA/cm.sup.2 Deliver high power density Areal capacity 0.1-5 mAh/cm.sup.2 Deliver high energy density 500 cycles (0.1 mAh/cm.sup.2) Long cycle life Cycle 290 cycles (0.5 mAh/cm.sup.2) Long cycle life life 133 cycles (1 mAh/cm.sup.2)   Ionic conductivity 2 mS/cm At room temperature Maximum of 2.6 mS/cm 0.5M Mg(OTf).sub.2 + 1M TBAC/monoglyme Plating potential −0.15 V vs. Mg/Mg.sup.2+ Low overpotential Stripping potential 0.17 vs. Mg/Mg.sup.2+ Low overpotential Anodic stability 4.0 V vs. Mg/Mg.sup.2+ High anodic stability on Pt electrode Morphology of Mg Homogeneous, dendrite-free Safe operation deposition (FIG. 16)

[0169] The electrolyte consisting of Mg(OTf).sub.2 salt and TBAC additive in ether solvents demonstrated excellent performance in Mg//Al—C asymmetric cell tests. The Mg//Al—C cell demonstrated high average Coulombic efficiency of 97.5% over 500 cycles, when operated at a current density of 0.5 mA/cm.sup.2 and areal capacity of 0.1 mAh/cm.sup.2 (FIG. 7b and FIG. 8b).

[0170] The Mg//Al—C cells were also cycled at various current densities and areal capacities to evaluate their robustness. High current densities translated to high power density, while high areal capacities translated to high energy density. The cells demonstrated excellent rate capability with current density up to 4 mA/cm.sup.2 and maintained Coulombic efficiency above 98% (FIG. 12a). The Mg//Al—C cells also demonstrated cycling at very high areal capacity up to 5 mAh/cm.sup.2 (FIG. 12b). These features are important to develop a new Mg-ion battery with high power density and high energy density, respectively.

[0171] The cycle life of Mg//Al—C cells is significantly dependent on the areal capacity of the plating/stripping process (FIG. 14). At an areal capacity of 0.1 mAh/cm.sup.2, the cell showed high Coulombic efficiency and maintained stability over 500 cycles. At higher areal capacities of 0.5 mAh/cm.sup.2 and 1 mAh/cm.sup.2, the cycle life of Mg//Al—C cells was reduced to 290 and 133 cycles, respectively. This was due to the severe degradation of Mg anode under high areal capacity cycling.

[0172] Mg(OTf).sub.2 and MgCl.sub.2 in Ether Solvent

[0173] The four electrolyte compositions which form clear solutions (Table 1) were examined. The experiments were conducted to evaluate the key technical performance of high-performance electrolyte systems, including Mg plating/stripping Coulombic efficiency and cycle life under different cycling conditions. Here, we present the electrochemical performance of the electrolyte combination of Mg(OTf).sub.2 and MgCl.sub.2 in monoglyme solvent. Electrolyte combinations based on other solvents are obtainable by controlling [Mg.sup.2+]:[Cl.sup.−] ratio in an ether solvent or mixtures of ether solvent. Table 5 summarizes the key technical features of the designed electrolyte formula.

TABLE-US-00005 TABLE 5 The key technical features of 0.3M Mg(OTf).sub.2 + 0.2M MgCl.sub.2 in monoglyme electrolyte. Key Features Performance Remarks Mg plating/stripping CE 99.4% (0.5 mA/cm.sup.2 and 0.1 Very high Coulombic (average) mAh/cm2, 1000 cycles) efficiency Current density 0.5-2.5 mA/cm.sup.2 Deliver high power density Areal capacity 0.1-5 mAh/cm.sup.2 Deliver high energy density Cycle 1000 cycles (0.1 mAh/cm.sup.2) Long cycle life life  115 cycles (0.5 mAh/cm.sup.2) Ionic conductivity 0.34 mS/cm At room temperature Maximum of 0.53 mS/cm 0.6M Mg(OTf).sub.2 + 0.4M MgCl.sub.2 in monoglyme Plating potential −0.17 V vs. Mg/Mg.sup.2+ Low overpotential Stripping potential 0.17 vs. Mg/Mg.sup.2+ Low overpotential Anodic stability 3.5 V vs. Mg/Mg.sup.2+ Good anodic stability on Pt electrode Morphology of Mg Homogeneous, dendrite-free Safe operation deposition (FIG. 17)

[0174] The electrolyte combination of Mg(OTf).sub.2 salt and MgCl.sub.2 in monoglyme demonstrated excellent performance in Mg//Al—C asymmetric cell tests. The combination of 0.3 M Mg(OTf).sub.2 and 0.2 M MgCl.sub.2 in monoglyme was found to be the optimal formula for Mg plating/stripping process. The Mg//Al—C cell demonstrated high average Coulombic efficiency of 99.4% over 1000 cycles at a current density of 0.5 mA/cm.sup.2 and areal capacity of 0.1 mAh/cm.sup.2 (FIG. 8c).

[0175] The Mg//Al—C cells were also cycled at various high current densities and high areal capacities to evaluate their robustness for practical application. High current densities translated to high power density, while high areal capacities translated to high energy density. The cells demonstrated excellent rate capability with current density up to 2.5 mA/cm.sup.2 and maintained Coulombic efficiency above 98% (FIG. 13a). The Mg//Al—C cells also demonstrated a good cycling performance at very high areal capacity up to 5 mAh/cm.sup.2 (FIG. 13b) with the Coulombic efficiency of 99.4%. These features are important to develop a new Mg-ion battery with high power density and high energy density, respectively.

Example 4. Morphology of the Electrodes

[0176] Mg(OTf).sub.2 and EMImCl in Ether Solvent

[0177] Examination of Mg deposition film on Al—C electrode revealed uniform and non-dendritic morphologies even at high areal capacity (1 mAh/cm.sup.2) before and after Mg deposition (FIG. 15a and FIG. 15b). This is a sharp contrast to highly dendritic lithium deposit morphologies. The use of Mg(OTf).sub.2-EMImCl electrolyte resulted in homogeneous Mg deposition on Al—C electrode and therefore reduced short-circuit of Al—C//Mg cell caused by Mg dendrite growth.

[0178] Mg(OTf).sub.2 and TBAC in Ether Solvent

[0179] Non-dendritic Mg deposition is an important criterion for safe battery operation. The morphology of a deposited Mg film was examined using scanning electron microscopy (SEM) (FIGS. 16a and 16b). The magnesium deposits showed non-dendritic crystalline particles associated with uniform metal deposition.

[0180] Mg(OTf).sub.2 and MgCl.sub.2 in Ether Solvent

[0181] Non-dendritic Mg deposition is also an important criterion for safe battery operation. The morphology of a deposited Mg film was examined using scanning electron microscopy (SEM) (FIGS. 17a and 17b). The magnesium deposits showed non-dendritic crystalline particles associated with uniform metal deposition.

Example 5. Electrochemical Properties Tested with Symmetric Cell

[0182] Mg(OTf).sub.2 and EMImCl in Ether Solvent

[0183] Symmetric Mg//Mg cells were employed to further investigate the stability of the Mg metal anode in the 0.5 M Mg(OTf).sub.2+0.6 M EMImCl in monoglyme electrolyte at high areal capacity (FIG. 18). Mg//Mg cells showed reversible plating/stripping processes at 0.5 mAh/cm.sup.2 (FIG. 18a) and 1 mAh/cm.sup.2 (FIG. 18b) with overpotential of ±0.35 V and ±0.45 V, respectively. This result consolidates the superior performance of Mg(OTf).sub.2 based electrolytes.

[0184] Mg(OTf).sub.2 and TBAC in Ether Solvent

[0185] The Mg//Mg symmetric cell also demonstrated excellent cycling performance up to 400 cycles or 800 h (FIG. 19). Reversible plating and stripping were clearly observed near −0.15 V and 0.15 V vs. Mg/Mg.sup.2+, respectively. The plating/stripping voltage increased only slightly with cycle number, indicating a stable Mg anode interphase in Mg(OTf).sub.2-based electrolyte with TBAC additive.

[0186] Mg(OTf).sub.2 and MgCl.sub.2 in Ether Solvent

[0187] The Mg//Mg symmetric cell also demonstrated excellent cycling performance up to 250 cycles or 500 h (FIG. 20). Reversible plating and stripping in symmetric cell were clearly observed near −0.18 V and 0.18 V vs. Mg/Mg.sup.2+, respectively. The plating/stripping voltage only increased slightly with cycle number, indicating a stable Mg anode interphase in Mg(OTf).sub.2-MgCl.sub.2 based electrolyte.

INDUSTRIAL APPLICABILITY

[0188] The disclosed electrolyte may be used in electrochemical cells, particularly magnesium ion batteries. As such electrolytes allow efficient plating and stripping of magnesium from a working electrode, such electrolytes may be used for the fabrication and assembly of magnesium-ion batteries which may be used as energy sources in various electrical and electronic devices.

[0189] Due to its ease of manufacture, the electrolytes described herein may also be produced on an industrial scale for easy assembly of magnesium ion electrochemical cells, which may be used as an alternative energy storage system to presently available technologies.

[0190] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.